A Bit More Generic Writer MOnad

gistfile1.scala

case class Writer[A](value: A, diary: String) = {
   def flatMap[B](f: A => Writer[B]) = {
       f(value) match {
           case Writer(result, d) => Writer(result, diary + d)
       }
   }    
   def map[B](f: A => B]) = Writer(f(value), diary)
}